Page 245 - Clinical Small Animal Internal Medicine

21 Supraventricular Arrhythmias 213

have underlying primary cardiac disease, in the form of strategy. Unfortunately, brief ECG recordings obtained
VetBooks.ir dilated cardiomyopathy or degenerative mitral valve dis­ in an unfamiliar environment do not adequately reflect
the daily variations of heart rate. Heart rate is therefore
ease. Less commonly, “lone atrial fibrillation” occurs in
giant‐breed dogs with structurally normal hearts. Atrial
reference, the average heart rate of healthy dogs varies
fibrillation is rarely identified in cats but most cats with better assessed from a 24‐hour Holter recording. For
AF have underlying structural cardiac disease with atrial between 85 and 100 bpm. Conversely, it is approximately
enlargement. Many of these cats also show signs of con­ 120 bpm in dogs with “lone” AF, 155 bpm in dogs with
gestive heart failure. underlying cardiac and 200 bpm in dogs with congestive
heart failure.
Signalment
Therapy
The vast majority of dogs with AF are giant‐ and large‐
breed dogs. In the giant‐breed group, Irish wolfhounds, Treatment Strategy
mastiffs, Newfoundlands, rottweilers, and Great Danes A rate‐control strategy is usually applied to the treat­
are overrepresented, with a male predisposition. Cats ment of AF. This approach aims at slowing ventricular
with AF are usually older males. rate in response to the rapid supraventricular impulses
which are bombarding the AV node. Control of the num­
ber of impulses, which are able to conduct through the
History and Clinical Signs
AV node, will help in alleviation of clinical signs and pre­
Most dogs with “lone” atrial fibrillation are asymptomatic. vention of further deterioration of ventricular function.
Conversely, some dogs with compensated cardiac disease However, it does not terminate the arrhythmia.
and AF show signs of lethargy, exercise intolerance, and Rate‐control is based on using drugs to decrease the
rarely anorexia, cough or syncope. When AF occurs in the ability of the AV node to conduct impulses. The need for
presence of congestive heart failure, generalized weak­ pharmacologic control of the heart rate is determined
ness, dyspnea, cough, and abdominal distension from from the heart rate distribution obtained from a baseline
ascites are usually present. Cats frequently show signs of 24‐hour Holter recording. The effect of rate‐control
an underlying cardiac disease, including dyspnea and drugs is then assessed on a follow‐up Holter two weeks
arterial thromboembolism. However, AF can also be an after treatment initiation. What constitutes adequate
incidental finding on auscultation in this species. rate control has not been precisely defined and may
depend on the animal’s underlying myocardial function.
Usually, an average heart rate of no greater than 120–
Diagnosis 140 bpm is considered adequate to observe clinical
Physical Examination improvement. If the average heart rate is considered too
On physical examination, AF is identified as a sustained high, drug dosages are increased by small increments
and irregularly irregular rhythm accompanied with pulse with close monitoring of the animal’s clinical status, as
deficits. A murmur may be ausculted, as severe underly­ response to antiarrhythmics varies between patients. In
ing cardiac disease is common in dogs with AF. contrast, medical therapy is usually not needed in dogs
with “lone” AF. “Lone” AF can be successfully terminated
Electrocardiography via DC electrical cardioversion. However, there does not
On the surface ECG, AF is an irregular tachyarrhythmia, appear to be any benefit in converting the rhythm when
with usually narrow QRS complexes and no P‐waves that can adequate/appropriate rate‐control therapy is effective.
be replaced by an undulation of the baseline, or F‐waves. It is
also permanent, indicating that sinus rhythm never sponta­ Medical Therapy
neously resumes following initiation of AF. The average heart Calcium Channel Blockers
rate is typically above 180–200 bpm (Figure 21.7). Diltiazem is widely used for the management of AF. A
The rate is much lower with “lone” AF, usually graded dose of diltiazem results in a decrease in ven­
approaching 100–120 bpm at rest. Although atrial fibril­ tricular response rate, because of slower action potential
lation is a SVT, concurrent lesions of the ventricular con­ propagation in the AV node. Oral formulations include
duction system (bundle branch block) may result in an extended‐release form. When evaluated for long‐
widening of the QRS complexes that may resemble those term management of AF, a dose of 3–5 mg/kg q12h PO
of ventricular tachycardia. of the extended‐release formulation usually achieves
satisfactory rate‐control. Better rate‐control, especially
Ambulatory 24‐Hour Holter Recording during periods associated with high adrenergic tone,
Determination of the average ventricular rate during AF can be obtained by adding digoxin to the treatment regi­
is critical information in deciding upon a treatment men. Calcium channel blockers decrease ventricular